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Music

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Narrator: Just to the east of M27, the "Apple Core" nebula,

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there's a star that can be seen in binoculars. And this star hosts

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a giant planet. The star is slightly smaller and

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cooler than our sun. In 2005, astronomers found it's Jupiter-

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size planet, named HD 189733b.

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Incredibly, the planet orbits just 3 million miles

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from the star, twelve times closer than Mercury orbits our sun.

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It's so close that the planet whips around its star in a little

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over two days. But this planet pays a steep price for skirting

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its sun. Sometimes, the star erupts with powerful flares,

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flares that heat the planet's upper atmosphere so much that the gasses

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simply escape. Since the planet passes in front if its

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star as we see it from Earth, astronomers were able to use the Hubble

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Space Telescope to establish that the atmosphere was evaporating.

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But when they looked again in 2010, there was not trace of an escaping

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atmosphere. Yet when astronomers looked once more in 2011,

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they saw dramatic evidence that the atmosphere was eroding.

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Hydrogen gas was rushing away from the planet at speeds over 300,000 miles

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an hour. What had changed? The same astronomers

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were also watching the star with NASA's Swift satellite. Just 8 hours before

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Hubble was scheduled to look for the planet's atmosphere, Swift saw the star

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erupt in a powerful X-ray flare. Similar

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flares happen frequently on the sun. But because the planet is so

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big and so close to its star, this X-ray blast had an outsized

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effect, heating the planet's atmosphere and sweeping away 1,000

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tons of gas each second. The escaping gas gave the planet a

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comet-like tail. While HD 189733b

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has plenty of gas to spare, atmospheric erosion is an

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important process, one able to whittle down any gas giant planet that

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hugs its star too close.